Giant eruptions from the Sun

By Nancy Crooker
Boston University

This solar eclipse was photographed in India
on February 16, 1980. Image courtesy the High Altitude Observatory
of the National Center for Atmospheric Research.

Should a modern Joshua stop the moon in its tracks during a total solar eclipse
so that for hours on end we could watch the hazy corona surrounding the
Sun, we would see an amazing phenomenon called a "coronal mass ejection"-CME,
for short. We would see a huge bubble of coronal material form and then
leave the Sun, heading out into space. We would have witnessed the birth
of a space storm.

CMEs were recognized as a common solar phenomenon in the 1970s when a technological
Joshua called a "coronagraph" flying on an orbiting laboratory called
"Skylab" held an artificial moon fixed in front of the Sun and inaugurated
an era of routine CME observations. A coronagraph simulates a total solar
eclipse by blocking the bright light from the Sun with a disk. To see
CMEs, a coronagraph must be above the atmosphere, where the sky is black,
as it is at ground level during a solar eclipse, so that the bright blue
light of the atmosphere does not drown out the light from the CMEs.

A CME captured by the LASCO coronagraph onboard
the SOHO spacecraft.
Image courtesy of SOHO/LASCO consortium. SOHO
is a project of international cooperation between ESA and NASA.

Since routine CME observations began, it has taken scientists years more of
research to establish that CMEs produce space storms, and to significantly
improve means of predicting them. Understanding and predicting space storms
is important to society because of our rapidly increasing dependence upon
technology that space storms affect. Space storms can cripple satellites,
interrupt communications systems, and cause power failures. We now understand
that the key storm ingredient in a CME is its magnetic field, which is
coiled inside it like a slinky. To appreciate why coiled magnetic fields
bring stormy space weather, one needs to know something about fair weather
in space-namely, the background flow called the "solar wind" and the configuration
of its imbedded magnetic fields.

Interplanetary space is filled with the solar wind, the Sun's expanding atmosphere
of high-speed charged particles, which sets the fair weather backdrop
for CMEs. Unlike weather on Earth, however, the important parameter is
not the wind itself but its imbedded magnetic field, which comes from
the Sun. Eugene Parker (Univ. of Chicago), the scientist who predicted
the existence of the solar wind, recognized that the solar wind would
draw the solar field out into space as if it were frozen to the flow.
Further, he deduced that in the ecliptic plane (that is, the plane in
which the planets rotate about the Sun), the Sun's rotation about its
own axis would cause the field lines to radiate away from the Sun in a
spiral pattern like water from a rotating garden sprinkler. Subsequent
spacecraft observations have proven Parker correct.

For Earth, the gently spiraling magnetic fields carried by the solar wind
herald fair weather because Earth's magnetic shield, the magnetosphere,
deflects them. The magnetosphere is a tadpole-shaped obstacle in the solar
wind created by Earth's magnetic field. At the head of the tadpole, where
it faces into the wind, Earth's magnetic field points northward, perpendicular
to the spiral field. Fields perpendicular to each other, for the most part,
slip past each other. Antiparallel fields, on the other hand, connect when
they meet. Thus any southward pointing fields in the solar wind link with
Earth's field, and like newly connected pipes, allow solar wind particles
and energy to pour into the magnetosphere. The magnetosphere's response
is a geomagnetic storm, complete with auroral displays and potential problems
for satellites, power grids, and radio transmission. What heralds bad space
weather for Earth, then, are southward fields in the solar wind-and the
stronger they are, the worse the storm.

What turns CMEs into space storms, then, is their strong, coiled fields,
which nearly guarantee some portions pointing southward. Moreover, for
those CMEs that travel faster than the background wind, the background
fields ahead of them become strong through compression, strengthening
any southward fluctuations there.

A halo CME recorded by the LASCO coronagraph
onboard the SOHO
spacecraft. Click on the image to see a movie of this event.
Image courtesy of SOHO/LASCO consortium. SOHO
is a project of international cooperation between ESA and NASA.

Viewed from the front, a CME in a coronagraph looks like a uniform halo
around the disk blocking the Sun, hence the name, "halo CME." Halo CMEs
have been difficult to detect because they are not as bright as CMEs viewed
from the side. However, recent technological advances have made them routinely
detectable with the coronagraph on the SOHO spacecraft. This ability will
be a boon for space weather predictions. In a retrospective study of data
from December 1996 to June 1997, nine halo CMEs were detected, and all
nine produced geomagnetic storms. Only three additional storms occurred
which could not be associated with halo CMEs.

This strong correspondence is remarkable in view of past prediction capabilities.
Traditionally solar flares have been used to predict storms, and the failure
rate has been high. Flares are brightening events on the Sun primarily
at wavelengths shorter than visible light. Viewed in x-rays, for example,
flares make the Sun look like a constantly sparkling jewel because they
occur so frequently. The brightest flares have been used for storm predictions,
and these often do accompany CMEs; hence their predictive capability.
But many CMEs occur with no apparent associated flares or with flares
so weak that they go undetected in standard monitoring techniques. Now
that we can see the CMEs themselves, there is no need to depend upon such
an unreliable predictor as a flare.

From an ethereal sight to a source of technological problems, CMEs continue
to fascinate scientists. How they form, why they occur, and what makes
some of them very fast are walls in our understanding that have yet to
come tumbling down.